1. Field of the Invention
The present invention relates to a state transition circuit that changes states consecutively, and synchronously with a signal, and varies the contents of control processes according to a state at every time instant; that is, a so-called state machine, and to a communication terminal that uses the state machine to control communication.
2. Description of the Related Art
It is a matter of common practice to construct a circuit responsible for various kinds of control by combining gates and latches. The circuit is used for control systems that are not as complex as a microcomputer and that require instantaneous control. However, the circuit may be used in combination with a microcomputer. Specifically, for example, overall processing is assigned to a CPU in a microcomputer and some control is assigned to the state transition circuit.
A control method can be such that state transitions are made consecutively with the progress of control. Even if the same signal is fed, the contents of control processes vary according to a state at every time instant. The method in which state transitions are made consecutively with the progress of control is referred to as a state transition method. The foregoing control circuit that is composed of gates and latches and makes state transitions is referred to as a state machine.
In recent years, digital mobile communications have become popular. In a terminal for mobile communication, a state machine is used to control transmission and reception. Sequences directly related to transmission and reception are required to be processed quickly. When such sequences are assigned to a microcomputer, the microcomputer must monitor the states of a receiver and a transmitter all the time. This results in the degraded performance of the microcomputer. From this viewpoint, the sequences are usually assigned to the state machine so that the microcomputer can concentrate on other processing.
A conventional state machine includes a state latch for storing data representing a state at a certain time instant, and a state control circuit for producing data representing a subsequent state according to the state at a certain time instant and a detection signal. When predetermined conditions are met in a certain state, the state machine makes a transition to another state. When other predetermined conditions are met in current state, the state machine makes a transition to yet another state. Thus, the state machine makes a transition to any given state.
Recently, there is an increasing demand for portable telephones. Above all, a cellular system is employed in the field of digital portable telephone service. A radio-frequency band usable within a cell that is a geographical region defined under the cellular concept is divided into narrower frequency bands. A portable terminal uses one of the frequency bands to transfer control data or voice and information. The data path through which data is transferred includes a searching BCCH channel, control channel, and a communication channel. The frequency employed differs from channel to channel. For realizing continuous communication while moving, a base station that is communicating may be switched to another one. Thus, frequencies are switched.
In vehicle-mounted mobile phone systems, digitization is in progress and a time division multiple access (hereinafter TDMA) method has been adopted widely. According to the TDMA method, data transfer is achieved in units of block data that is data having a length of a given number of bytes. In general, block data to be handled at a lower level (for example, in a physical layer) of transfer described in a protocol includes control data, concerning a high layer of transfer described in a protocol, and a message. The data length of data in a high layer of transfer described in a protocol may be smaller than that of block data such as the K-th block data. In this case, a general protocol stipulates that if no effective data is handled in a high layer of transfer described in the protocol, block data should be padded with a specific fill pattern (for example, 0s).
A terminal included in a vehicle-mounted mobile phone system comprises a state machine, a receiver, a transmitter, a phase-locked loop (hereinafter PLL), and a computer. Among the components, each of the state machine, receiver, transmitter, and PLL is usually realized with a single LSI. Alternatively, the state machine, receiver, and transmitter may be realized with a single LSI.
For testing an LSI that realizes a state machine or an LSI that realizes the state machine, receiver, and transmitter, the state machine that can assume a plurality of states need not be tested in all the states thereof. Whether or not the state machine operates normally can be checked merely by testing it in a specific state. However, when it is required to test the state machine in a certain state, the state machine must be reset to the initial state, and changed from one state to another until it is placed in the test state. In other words, the state machine cannot be changed to a desired state until it satisfies the conditions for all preceding states. When a state machine that makes complex state transitions is concerned, it takes much time to place the state machine in a desired state. This leads to degradation in the use efficiency of a tester and an increase in test cost.
When given conditions are met, a state machine makes a transition to another state. As far as testing is concerned, it may be required to retain the state machine in a certain state. For retaining a state machine in a conventional terminal in a certain state, setting performed by an internal logic circuit or designated at an external unit is controlled so that the conditions for another state will not be met. However, this control is complex. It is therefore demanded that a certain state can be retained effortlessly.
In the foregoing communication terminal, a microcomputer is designed to determine whether a PLL is locked to a frequency during frequency setting or to mask a state machine until a frequency is locked as a result of frequency switching. This means that a microcomputer must bear a heavy load, leading to a lower processing speed. When a microcomputer offering superb processing ability is used in order to solve this problem, cost increases.
Likewise, in a conventional transmitter for transmitting block data, a microcomputer is designed to write fill pattern in a data register. This also means that a microcomputer must bear a heavy load. The foregoing problems ensue.